The present invention relates to wave field synthesis systems, and, in particular, to the aliasing correction in wave field synthesis systems.
There is an increasing need for new technologies and innovative products in the field of entertainment electronics. It is an important prerequisite for the success of new multimedia systems to offer optimal functionalities or capabilities. This is achieved by the employment of digital technologies and, in particular, computer technology. Examples for this are the applications offering an enhanced close-to-reality audiovisual impression. In previous audio systems, a substantial disadvantage lies in the quality of the spatial sound reproduction of natural, but also of virtual environments.
Methods of multi-channel loudspeaker reproduction of audio signals have been known and standardized for many years. All usual techniques have the disadvantage that both the site of the loudspeakers and the position of the listener are already impressed on the transfer format. With wrong arrangement of the loudspeakers with reference to the listener, the audio quality suffers significantly. Optimal sound is only possible in a small area of the reproduction space, the so-called sweet spot.
A better natural spatial impression as well as greater enclosure or envelope in the audio reproduction may be achieved with the aid of a new technology. The principles of this technology, the so-called wave field synthesis (WFS), have been studied at the TU Delft and first presented in the late 80s (Berkout, A. J.; de Vries, D.; Vogel, P.: Acoustic control by Wave field Synthesis. JASA 93, 993).
Due to this method's enormous demands on computer power and transfer rates, the wave field synthesis has up to now only rarely been employed in practice. Only the progress in the field of the microprocessor technology and the audio encoding do permit the employment of this technology in concrete applications today. First products in the professional field are expected next year. In a few years, first wave field synthesis applications for the consumer area are also supposed to come on the market.
The basic idea of WFS is based on the application of Huygens' principle of the wave theory:
Each point caught by a wave is starting point of an elementary wave propagating in spherical or circular manner.
Applied on acoustics, every arbitrary shape of an incoming wave front may be replicated by a large amount of loudspeakers arranged next to each other (a so-called loudspeaker array). In the simplest case, a single point source to be reproduced and a linear arrangement of the loudspeakers, the audio signals of each loudspeaker have to be fed with a time delay and amplitude scaling so that the radiating sound fields of the individual loudspeakers overlay correctly. With several sound sources, for each source the contribution to each loudspeaker is calculated separately and the resulting signals are added. If the sources to be reproduced are in a room with reflecting walls, reflections also have to be reproduced via the loudspeaker array as additional sources. Thus, the expenditure in the calculation strongly depends on the number of sound sources, the reflection properties of the recording room, and the number of loudspeakers.
In particular, the advantage of this technique is that a natural spatial sound impression across a great area of the reproduction space is possible. In contrast to the known techniques, direction and distance of sound sources are reproduced in a very exact manner. To a limited degree, virtual sound sources may even be positioned between the real loudspeaker array and the listener.
Although the wave field synthesis functions well for environments the properties of which are known, irregularities occur if the property changes or the wave field synthesis is executed on the basis of an environment property not matching the actual property of the environment.
The technique of the wave field synthesis, however, may also be advantageously employed to supplement a visual perception by a corresponding spatial audio perception. Previously, in the production in virtual studios, the conveyance of an authentic visual impression of the virtual scene was in the foreground. The acoustic impression matching the image is usually impressed on the audio signal by manual steps in the so-called postproduction afterwards or classified as too expensive and time-intensive in the realization and thus neglected. Thereby, usually a contradiction of the individual sensations arises, which leads to the designed space, i.e. the designed scene, to be perceived as less authentic.
In the technical publication “Subjective experiments on the effects of combining spatialized audio and 2D video projection in audio-visual systems”, W. de Bruijn and M. Boone, AES convention paper 5582, May 10 to 13, 2002, Munich, subjective experiments with reference to effects of combining spatial audio and a two-dimensional video projection in audiovisual systems are illustrated. In particular, it is stressed that two speakers standing at differing distance to a camera and almost standing behind each other can be better understood by a viewer if the two people standing behind each other are seen and reconstructed as different virtual sound sources with the aid of the wave field synthesis. In this case, by subjective tests, it has turned out that a listener can better understand and distinguish the two speakers, who are talking at the same time, separately from each other.
In the audio field, by the technique of the wave field synthesis (WFS), good spatial sound for a large listener area can be accomplished. As it has been set forth, the wave field synthesis is based on the Huygens principle, according to which wave fronts may be shaped and built up by superimposition of elementary waves. According to a mathematically exact, theoretical description, an infinite number of sources in infinitely small distance would have to be used for the generation of the elementary waves. In practice, however, a finite number of loudspeakers is used in a finite, small distance to each other. Each of these loudspeakers is controlled with an audio signal from a virtual source having a certain delay and a certain level, according to the WFS principle. Levels and delays are usually different for all loudspeakers.
At is has already been set forth, the wave field synthesis system works on the basis of the Huygens principle and reconstructs a given waveform, for example, of a virtual source arranged at a certain distance to a show area or a listener in the show area by a multiplicity of individual waves. The wave field synthesis algorithm thus obtains information on the actual position of an individual loudspeaker from the loudspeaker array to then calculate, for this individual loudspeaker, a component signal this loudspeaker then finally has to irradiate, so that a superimposition of the loudspeaker signal from the one loudspeaker with the loudspeaker signals of the other active loudspeakers performs a reconstruction in that the listener has the impression that he or she is not “irradiated with sound” by many individual loudspeakers, but only by a single loudspeaker at the position of the virtual source.
For several virtual sources in a wave field synthesis setting, the contribution of each virtual source for each loudspeaker, i.e. the component signal of the first virtual source for the first loudspeaker, of the second virtual source for the first loudspeaker, etc., is calculated to then add the component signals to finally obtain the actual loudspeaker signal. In case of, for example, three virtual sources, the superimposition of the loudspeaker signals of all active loudspeakers at the listener would lead to the listener not having the impression that he or she is irradiated with sound from a large array of loudspeakers, but that the sound he or she is hearing only comes from three sound sources positioned at special positions, which are equal to the virtual sources.
In practice, the calculation of the component signals mostly takes place by the audio signal associated with a virtual source being imparted with a delay and a scaling factor at a certain time instant, depending on position of the virtual source and position of the loudspeaker, in order to obtain a delayed and/or scaled audio signal of the virtual source, which immediately represents the loudspeaker signal, when only one virtual source is present, or which then contributes to the loudspeaker signal for the loudspeaker considered, after addition with further component signals for the loudspeaker considered from other virtual sources.
Typical wave field synthesis algorithms work independently of how many loudspeakers are present in the loudspeaker array. The theory underlying the wave field synthesis consists in the fact that each arbitrary sound field may be exactly reconstructed by an infinitely high number of individual loudspeakers, the individual loudspeakers being arranged infinitely close to each other. In practice, however, neither the infinitely high number nor the infinitely close arrangement can be realized. Instead, there are a limited number of loudspeakers, which are additionally arranged in certain given distances to each other. With this, in real systems, only an approximation is achieved to the actual waveform that would take place if the virtual source was actually present, i.e. was a real source.
Due to loudspeaker array effects, a summation of the bass portions of 3 dB per octave, for example, occurs below an aliasing frequency. This amplification is a result of the sound wave superimpositions for basses in the WFS reproduction. For this reason, for the WFS reproduction below the aliasing frequency, a static filter correcting, i.e. lowering, the bass portion is calculated. This filter is calculated in dependence on the loudspeaker distance, and adjusting the aliasing frequency is currently made manually according to the auditory sensation of the sound monitoring operator.
It has been found that the manual adjustment is subjective and, thus, takes a lot of effort, and has further led to strong variations in the quality of the perceived tone.
In the technical publications by E. Corteel, U. Horbach, R. S. Pellegrini: “Multichannel Inverse Filtering of Multiexciter Distributed Mode Loudspeakers for Wave Field Synthesis”, AES convention paper 5611, May 10-13, Munich, and U. Horbach, E. Corteel, D. de Vries: “Spatial Audio Reproduction using Distributed Mode Loudspeaker Arrays”, AES conference paper, June 1-3, St. Petersburg, as well as the patent DE 103 21 986 relate to amplitudes, or a frequency manipulation, for quality improvements in the wave field synthesis.